1. Field of the Invention
The present invention relates generally to air cooled turbine airfoils, and more specifically to the cooling of a turbine airfoil trailing edge.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, a turbine section includes a plurality of stages of stator vanes and rotor blades to convert chemical energy from a hot gas flow into mechanical energy by driving the rotor shaft. The engine efficiency can be increased by passing a higher gas flow temperature through the turbine section. The maximum temperature passed into the turbine is determined by the first stage stator vanes and rotor blades.
These turbine airfoils (stator vanes and rotor blades) can be designed to withstand extreme temperatures by using high temperature resistant super-alloys. Also, higher temperatures can be used by providing internal convection cooling and external film cooling for the airfoils. Complex internal cooling circuits have been proposed to maximize the airfoil internal cooling while using a minimum amount of pressurized cooling air to also increase the engine efficiency.
Besides allowing for a higher external temperature, cooling of the airfoils reduces hot spots that occur around the airfoil surface and increase the airfoil oxidation and erosion that would result in shorter part life. This is especially critical in an industrial gas turbine engine where operation times hot between engine start-up and shut-down is from 24,000 to 48,000 hours. Unscheduled engine shut-down due to a damaged part such as a turbine airfoil greatly increases the cost of operating the engine.
Airfoils constructed with cavities and passageways for carrying cooling fluid there through are well known in the art. For example, it is common to construct airfoils with spanwise cavities within the wider forward portion. These cavities often have inserts disposed therein which define compartments and the like within the cavities. The cooling fluid is brought into the cavities and compartments and some of the fluid is often ejected there from via holes in the airfoil walls to film cool the external surface of the airfoil. The trailing edge region of airfoils is generally more difficult to cool than other portions of the airfoil because the cooling air is hot when it arrives at the trailing edge since it has been used to cool other portions of the airfoil, and the relative thinness of the trailing edge region limits the rate at which cooling fluid can be passed through that region.
A common technique for cooling the trailing edge region is to pass cooling fluid from the larger cavity in the forward portion of the airfoil through the trailing edge region of the airfoil via a plurality of small diameter drilled passageways. Such an airfoil construction is shown in U.S. Pat. No. 4,183,716 issued to Takahara et al on Jan. 15, 1980 and entitled AIR-COOLED TURBINE BLADE. Another common technique for convectively cooling the trailing edge region is by forming a narrow slot between the walls in the trailing edge region and having the slot communicate with a cavity in the forward portion of the airfoil and with outlet means along the trailing edge of the airfoil. The slot carries the cooling fluid from the cavity to the outlets in the trailing edge. An array of pedestals extending across the slot from the pressure to the suction side wall are typically incorporated to create turbulence in the cooling air flow as it passes through the slot and to increase the convective cooling surface area of the airfoil. The rate of heat transfer is thereby increased, and the rate of cooling fluid flow required to be passed through the trailing edge region may be reduced.
Another airfoil constructed with improved means for carrying cooling fluid from a cavity in the forward portion of the airfoil through the trailing region and out the trailing edge of the airfoil is shown in U.S. Pat. No. 4,203,706 issued to Hess on May 20, 1980 and entitled RADIAL WAFER AIRFOIL CONSTRUCTION. In that patent wavy criss-crossing grooves in opposing side walls of the trailing edge region provide tortuous paths for the cooling fluid through the trailing edge region and thereby improve heat transfer rates.
Another prior art airfoil with a trailing edge cooling passage is U.S. Pat. No. 3,819,295 issued to Hauser et al on Jun. 25, 1974 and entitled COOLING SLOT FOR AIRFOIL BLADE which discloses an intersecting arrangement of cooling passages formed by turbulators extending from the side walls of the trailing edge of the airfoil to promote turbulence in the cooling air passing through the trailing edge.
In U.S. Pat. No. 4,407,632 issued to Liang on Oct. 4, 1983 and entitled AIRFOIL PEDESTALED TRAILING EDGE REGION COOLING CONFIGURATION, the airfoil trailing edge region is cooled by a plurality of slots formed between the pressure and suction side walls with an array of pedestals extending across the slot such that the cooling air snakes around the pedestals in a spiral-like or vortex-like flow path to improve the heat transfer from the hot airfoil wall to the cooling air.
Despite the variety of trailing edge region cooling configurations described in the prior art, further improvement is always desirable in order to allow the use of higher operating temperatures, less exotic materials, and reduced cooling air flow rates through the airfoils, as well as to minimize manufacturing costs.
An object of the present invention is to provide for a turbine airfoil with an improved convective cooling configuration in the trailing edge region.